Integrated process for the production of formaldehyde-stabilised urea

ABSTRACT

A process for the production of formaldehyde-stabilised urea is described comprising the steps of: (a) generating a synthesis gas comprising hydrogen, nitrogen, carbon monoxide, carbon dioxide and steam in a synthesis gas generation unit, (b) subjecting the synthesis gas to one or more stages of water-gas shift in one or more water-gas shift reactors to form a shifted gas; (c) recovering carbon dioxide from the shifted gas in a carbon dioxide removal unit to form a carbon dioxide-depleted synthesis gas; (d) synthesising methanol from the carbon dioxide-depleted synthesis gas in a methanol synthesis unit and recovering the methanol and a methanol synthesis off-gas comprising nitrogen, hydrogen and residual carbon monoxide; (e) subjecting at least a portion of the recovered methanol to oxidation with air in a formaldehyde production unit; (f) subjecting the methanol synthesis off-gas to methanation in a methanation reactor containing a methanation catalyst to form an ammonia synthesis gas; (g) synthesising ammonia from the ammonia synthesis gas in an ammonia production unit and recovering the ammonia; (h) reacting a portion of the ammonia and at least a portion of the recovered carbon dioxide stream in a urea production unit to form a urea stream; and (i) stabilising the urea by mixing the urea stream and a stabiliser prepared using formaldehyde recovered from the formaldehyde production unit, wherein a portion of the synthesis gas generated by the synthesis gas generation unit by-passes either the one or more water-gas shift reactors; the carbon dioxide removal unit; or the one or more water-gas shift reactors and the carbon dioxide removal unit.

The present invention relates to a process for the production offormaldehyde-stabilised urea. More particularly, it relates to anintegrated process for the production of formaldehyde-stabilised urea ina process including the co-production of methanol and ammonia.

Urea finds widespread use as a fertiliser and in industrial chemicalmanufacture. It is conventionally made by reacting ammonia with carbondioxide to form a solid product which is often shaped by prilling orgranulating. Formaldehyde or a urea-formaldehyde concentrate (UFC) areoften used to stabilise the urea before or during the shaping process.

However, the demand for formaldehyde to stabilise urea from a singleproduction facility is small and beyond the economic feasibility for adedicated formaldehyde stabiliser production facility. Due to the smallscale of the requirements, the formaldehyde is normally produced at aseparate dedicated formaldehyde stabiliser production facility andtransported to the ammonia/urea production facility where it is stored.

We have developed an integrated urea-formaldehyde process with adedicated formaldehyde stabiliser production unit based on amethanol-ammonia co-production process that provides improvedflexibility in the amount of methanol, ammonia and urea synthesised.

Accordingly, the invention provides a process for the production offormaldehyde-stabilised urea comprising the steps of: (a) generating asynthesis gas comprising hydrogen, nitrogen, carbon monoxide, carbondioxide and steam in a synthesis gas generation unit, (b) subjecting thesynthesis gas to one or more stages of water-gas shift in one or morewater-gas shift reactors to form a shifted gas; (c) recovering carbondioxide from the shifted gas in a carbon dioxide removal unit to form acarbon dioxide-depleted synthesis gas; (d) synthesising methanol fromthe carbon dioxide-depleted synthesis gas in a methanol synthesis unitand recovering the methanol and a methanol synthesis off-gas comprisingnitrogen, hydrogen and residual carbon monoxide; (e) subjecting at leasta portion of the recovered methanol to oxidation with air in aformaldehyde production unit; (f) subjecting the methanol synthesisoff-gas to methanation in a methanation reactor containing a methanationcatalyst to form an ammonia synthesis gas; (g) synthesising ammonia fromthe ammonia synthesis gas in an ammonia production unit and recoveringthe ammonia; (h) reacting a portion of the ammonia and at least aportion of the recovered carbon dioxide stream in a urea production unitto form a urea stream; and (i) stabilising the urea by mixing the ureastream and a stabiliser prepared using formaldehyde recovered from theformaldehyde production unit, wherein a portion of the synthesis gasgenerated by the synthesis gas generation unit by-passes either the oneor more water-gas shift reactors; the carbon dioxide removal unit; orthe one or more water-gas shift reactors and the carbon dioxide removalunit.

There have been numerous designs for ammonia and methanol co-productionover the last 50 years or so, but they have generally focussed ongenerating large quantities of both materials as saleable products.Examples of such processes are described, for example in U.S. Pat. No.6,106,793, U.S. Pat. No. 6,333,014, U.S. Pat. No. 7,521,483, U.S. Pat.No. 8,247,463, U.S. Pat. No. 8,303,923, and WO2013/102589. None of theseprocesses include a dedicated formaldehyde production unit as claimed.

In the claimed process, a portion of the synthesis gas generated by thesynthesis gas generation unit by-passes either the one or more water-gasshift reactors; the carbon dioxide removal unit; or the one or morewater-gas shift reactors and the carbon dioxide removal unit. Thestreams by-passing the one or more water-gas shift reactors are combinedwith the shifted gas stream fed to the carbon dioxide removal unit. Thestreams by-passing the carbon dioxide removal unit are combined with thecarbon-dioxide depleted gas stream fed to the methanol synthesis unit.Streams that bypass the carbon dioxide removal unit are desirablytreated to remove water therefrom to lessen the amount of water in thecrude methanol and improve the rate of the methanol synthesis reaction.The by-pass streams allow the levels of carbon monoxide and/or carbondioxide to the methanol synthesis unit to be controlled therebyincreasing the flexibility in the amount of methanol, ammonia and ureasynthesised.

In addition, the methanol synthesis reactions may be depicted asfollows:

CO+2H₂

CH₃OH

CO₂+3H₂

CH₃OH+H₂O

From these reactions, it can be seen that carbon dioxide makes water asa by-product in the methanol synthesis whereas carbon monoxide does not.If the major carbon oxide source for the methanol synthesis reaction iscarbon monoxide, less water will be formed in the reaction which has theadded advantage that the crude methanol need not be subjected to adistillation step before being used as feed to the formaldehyde plant,which offers considerable savings in capital and operating costs. If thecarbon-dioxide removal stage is by-passed and hence the proportion ofthe carbon dioxide in the methanol synthesis feed gas is higher, morewater is produced but the methanol synthesis reactor may be smaller.

In the process, the methanol converter for the formaldehyde productionis placed between the carbon dioxide removal and methanation stages ofthe ammonia/urea plant, where carbon oxide levels are low, preventingexcessive hydrogen consumption. There is still a need for a methanatorbecause, to maintain a reasonable sized methanol converter, the approachto equilibrium is kept relatively high.

Other possible placements of the methanol converter are upstream of thehigh temperature shift converter and immediately upstream of thesynthesis loop. In the first case, the temperatures are too high formethanol synthesis and the high levels of all reactants makesover-conversion possible, with associated wastage of hydrogen. In thelatter case, the methanol synthesis would need to run at ammoniasynthesis pressure (>130 bar) which requires non-conventional, and moreexpensive, methanol synthesis apparatus.

Should methanol reach the methanation unit, it will conveniently beconverted into carbon oxides and water. The carbon oxides will then bemethanated as normal. This process would generate a small endotherm tocompete against the exothermic methanation. An additional benefit ofthis scheme is clear when considering the two sets of reactions;

Methanation Methanol synthesis CO + 3H₂

 CH₄ + H₂O CO + 2H₂

 CH₃OH CO₂ + 4H₂

 CH₄ + 2H₂O CO₂ + 3H₂

 CH₃OH + H₂O

Any methanol synthesis that occurs saves one mole of hydrogen per moleof methanol produced (assuming equivalent consumption of carbon monoxideand dioxide), enabling increased ammonia production, estimated to beapproximately an additional third of a percent, equivalent to about 7mtpd on a 2,000 mtpd ammonia plant.

The synthesis gas, comprising carbon monoxide, carbon dioxide, hydrogenand nitrogen provided in step (a) may be formed by any suitable means.Different synthesis gas generation units can provide synthesis gaseswith different carbon monoxide:carbon dioxide ratios. The process allowsthe product mix to be adjusted for a wide range of synthesis gascompositions. The synthesis gas generation may be based on primary steamreforming of a hydrocarbon such as natural gas, naphtha or a refineryoff-gas and secondary reforming with air or oxygen-enriched air; or bythe gasification of a carbonaceous feedstock, such as coal or biomasswith air. Preferably the synthesis gas generation stage comprises steamreforming a hydrocarbon. This may be achieved by primary reforming ahydrocarbon with steam in externally-heated catalyst-filled tubes in afired- and/or gas-heated steam reformer and secondary reforming theprimary-reformed gas mixture in an autothermal or secondary reformer bysubjecting it to partial combustion with air, or air enriched in oxygen,and then passing the partially combusted gas mixture through a bed ofsteam reforming catalyst. A heat exchange reformer, such as a gas-heatedsteam reformer (GHR), may be operated in parallel with a conventionalfired reformer or in series with a conventional fired reformer and theproduct gas fed to a common secondary reformer. By-passing a portion ofthe hydrocarbon feedstock around a primary reformer may be used toreduce the carbon monoxide:carbon dioxide ratio in the synthesis gas. Ifdesired one or more stages of adiabatic pre-reforming may also beperformed before the fired reformer and/or heat exchange reformer.

The primary reforming catalyst typically comprises nickel at levels inthe range 5-30% wt, supported on shaped refractory oxides, such as alphaalumina, magnesium aluminate or calcium aluminate. If desired, catalystswith different nickel contents may be used in different parts of thetubes, for example catalysts with nickel contents in the range 5-15% wtor 30-85% wt may be used advantageously at inlet or exit portions of thetubes. Alternatively, structured catalysts, wherein a nickel or preciousmetal catalyst is provided as a coated layer on a formed metal orceramic structure may be used, or the catalysts may be provided in aplurality of containers disposed within the tubes. In a fired steamreformer, steam reforming reactions take place in the tubes over thesteam reforming catalyst at temperatures above 350° C. and typically theprocess fluid exiting the tubes is at a temperature in the range650-950° C. The combusting gases flowing around the outside of the tubesmay have a temperature in the range 900-1300° C. In a GHR, the catalystmay again be at temperatures above 350° C. with the process fluidexiting the tubes at a temperature in the range 500-950° C. Here, theheat exchange medium flowing around the outside of the tubes may have atemperature in the range 500-1200° C. The pressure may be in the range10-80 bar abs. In a secondary reformer, the primary-reformed gas ispartially combusted often in a burner apparatus mounted usually near thetop of the reformer. The partially combusted reformed gas is then passedadiabatically through a bed of a steam reforming catalyst usuallydisposed below the burner apparatus, to bring the gas compositiontowards equilibrium. Heat for the endothermic steam reforming reactionis supplied by the hot, partially combusted reformed gas. As thepartially combusted reformed gas contacts the steam reforming catalystit is cooled by the endothermic steam reforming reaction to temperaturesin the range 900-1100° C. The bed of steam reforming catalyst in thesecondary reformer typically comprises nickel at levels in the range5-30% wt, supported on shaped refractory oxides, but layered beds may beused wherein the uppermost catalyst layer comprises a precious metal,such as platinum or rhodium, on a zirconia support. Such steam reformingapparatus and catalysts are commercially available.

Alternatively, the steam reforming maybe achieved by passing a mixtureof the hydrocarbon and steam through an adiabatic pre-reformercontaining a bed of steam reforming catalyst and then passing thepre-reformed gas mixture and air to an autothermal reformer whichoperates in the same way as the secondary reformer to produce a gasstream containing hydrogen, carbon oxides and steam. In adiabaticpre-reforming, a mixture of hydrocarbon and steam, typically at a steamto carbon ratio in the range 1-4, is passed at an inlet temperature inthe range 300-620° C. to a fixed bed of pelleted nickel-containingpre-reforming catalyst. Such catalysts typically comprise ≥40% wt nickel(expressed as NiO) and may be prepared by co-precipitation of anickel-containing material with alumina and promoter compounds such assilica and magnesia. Again, the pressure may be in the range 10-80 barabs.

Alternatively, the reaction stream may be formed by gasification ofcoal, biomass or other carbonaceous material with air using gasificationapparatus. In such processes the coal, biomass or other carbonaceousmaterial is heated to high temperatures in the absence of a catalyst toform a crude synthesis gas often containing sulphur contaminants such ashydrogen sulphide, which have to be removed. Gasification ofcarbonaceous feedstock to produce a synthesis gas may be achieved usingknown fixed bed, fluidised-bed or entrained-flow gasifiers attemperatures in the range 900-1700° C. and pressures up to 90 bar abs.The crude synthesis gas streams require additional treatments known inthe art to remove unwanted sulphur and other contaminants.

In a preferred process, the synthesis gas generation stage comprisesprimary reforming a hydrocarbon, particularly natural gas, in a firedsteam reformer to produce a gas stream comprising hydrogen, carbonmonoxide, carbon dioxide and steam, and secondary reforming in which theprimary reformed gas is further reformed in a secondary reformer usingair or oxygen-enriched air to provide a synthesis gas stream comprisinghydrogen, carbon oxides and nitrogen.

If desired, one air feed may be provided for both the production of thesynthesis gas and the production of the formaldehyde. This offersbenefits in the reduction of capital and operating costs when comparedto that required for the separate systems utilised in the prior art. Ingenerating synthesis gas on ammonia plants, multiple stages ofcompression are often used. The air for the methanol oxidation maytherefore conveniently be taken after the first stage and before thefinal stage of air compression. This air source therefore removes theneed for a separate air compression unit for the formaldehyde productionunit. Thus in one embodiment, a single source of air is compressed,divided into first and second portions, the first portion provided to aformaldehyde production unit and the second portion further compressedand provided to a synthesis gas generation unit. The first portion ofcompressed air provided to the formaldehyde production unit is used tooxidise at least a portion of the methanol. The first portion may becompressed to a pressure in the range 1.1-5 bar abs. The second portionof compressed air fed to the synthesis gas generation unit is used togenerate the synthesis gas, for example in a secondary or autothermalreformer. The second portion may be compressed to 10-80 bar abs. Ifdesired, the second portion may also be preheated. The amount of air inthe second portion may also be varied to control the eventualhydrogen:nitrogen ratio in the ammonia synthesis gas. The proportion ofcompressed air fed to the formaldehyde production unit may be up toabout 20% by volume, preferably in the range 1.5-15% by volume, of thetotal air fed to the process.

Before recovery of the carbon dioxide, the crude synthesis gas issubjected in step (b) to one or more stages of water-gas shift toproduce a shifted synthesis gas with the desired gas composition. In awater-gas shift stage, a portion of the carbon monoxide in the stream isconverted to carbon dioxide. Any suitable catalytic shift conversionreactor and catalyst may be used. If insufficient steam is present,steam may be added to the gas stream before it is subjected to thewater-gas shift conversion. The reaction may be depicted as follows;

H₂O+CO

H₂+CO₂

The reaction may be carried out in one or more stages. The, or each,stage may be the same or different and may be selected from hightemperature shift, low temperature shift, medium temperature shift,isothermal shift and sour shift, and is preferably selected from asingle stage of high temperature shift, a combination of hightemperature shift and low temperature shift, a single stage of mediumtemperature shift, or a combination of medium temperature shift and lowtemperature shift.

High temperature shift catalysts may be promoted iron catalysts such aschromia- or alumina-promoted magnetite catalysts. Other high temperatureshift catalysts may be used, for example iron/copper/zinc oxide/aluminacatalysts, manganese/zinc oxide catalysts or zinc oxide/aluminacatalysts. Medium, low temperature and isothermal shift catalyststypically comprise copper, and useful catalysts may comprise varyingamounts of copper, zinc oxide and alumina. Alternatively, where sulphurcompounds are present in the gas mixture, such as synthesis gas streamsobtained by gasification, so-called sour shift catalysts, such as thosecomprising sulphides of molybdenum and cobalt, are preferred. Suchwater-gas shift apparatus and catalysts are commercially available.

For high temperature shift catalysts, the temperature in the shiftconverter may be in the range 300-460° C. or 300-360° C., for mediumtemperature shift catalysts the temperature may be in the range 190-300°C. and for low-temperature shift catalysts the temperature may be185-270° C. The water-gas shift reaction is exothermic and so coolingmay therefore be necessary between the stages. For sour shift catalyststhe temperature may be in the range 200-370° C. The flow-rate ofsynthesis gas containing steam may be such that the gas hourly spacevelocity (GHSV) through the bed of water-gas shift catalyst in thereactor may be ≥6000 hour⁻¹. The pressure may be in the range 10-80 barabs.

In a preferred embodiment, the water-gas shift stage comprises a hightemperature shift stage or a medium temperature shift stage or anisothermal shift stage with or without a low temperature shift stage.

The portion of the synthesis gas produced by the synthesis gasgeneration unit may by-pass one or more of the water-gas shift stages,each stage comprising one or more water-gas shift reactors. Thus theportion of the synthesis gas may by-pass a high temperature shiftreactor, a low temperature shift reactor, a medium temperature shiftreactor, which may be an isothermal medium temperature shift reactorcontaining a bed of cooled medium-temperature water gas shift catalyst,or any combination of these. In a preferred embodiment where the shiftstage consists of a high temperature shift reactor and a low temperatureshift reactor, the portion of the synthesis gas may by-pass either thehigh temperature shift reactor, the low temperature shift reactor orboth the high temperature shift reactor and the low temperature shiftreactor. A by-pass around just the low temperature shift reactor ispreferred because it produces a crude methanol stream with lower levelsof water.

The amount of the by-pass streams around the water-gas shift reactorsmay be adjusted depending on the synthesis gas composition and thedesired carbon monoxide/carbon dioxide content of the gas stream fed tothe methanol synthesis stage. For example, a by-pass around a hightemperature shift converter will likely be high in carbon monoxide andrelatively lower in carbon dioxide. The carbon monoxide concentration ofthe by-pass stream around the high temperature shift in such a case maybe in the range of 10-15 vol % on a dry basis and the carbon dioxideconcentration may be in the range of 5-10 vol % on a dry basis. A hightemperature shift reactor converts the bulk of the carbon monoxide inthe synthesis gas to carbon dioxide; hence the inlet composition of adownstream water-gas shift reactor, such as a low temperature shiftreactor, will be lower in carbon monoxide and higher in carbon dioxide.The carbon monoxide concentration inlet the downstream shift reactor maybe in the range of 2-5 vol % on a dry basis and the carbon dioxideconcentration may be in the range of 13-17 vol % on a dry basis. Thedownstream shift reactor converts still more carbon monoxide to carbondioxide, and a shifted gas stream, e.g. from a low temperature shiftreactor, may contain 0.1-0.5 vol % carbon monoxide on a dry basis and15-20 vol % carbon dioxide on a dry basis.

One aspect of this invention is to co-produce a relatively small amountof methanol for the purpose of making formaldehyde to stabilise ureaproduced from ammonia obtained from one or more ammonia plants.Therefore, a relatively small concentration of carbon oxides may besufficient to produce the required amount of methanol.

The amount of gas that is by-passed around each unit is dependent on therequired methanol production rate and the composition of the by-passstream. The higher the production rate of methanol, the higher therequired by-pass flow rates around one or more of the water-gas shiftreactors.

The unshifted synthesis gas is rich in carbon monoxide compared to theshifted synthesis gas, therefore the proportion of by-pass of unshiftedgas will be smaller. The carbon dioxide content of the synthesis gasdoes not affect the proportion of by-pass when the by-pass stream isre-combined with a shifted synthesis gas stream upstream of the carbondioxide removal unit. In a preferred embodiment where a by-pass streamof a partially shifted gas from a high or medium temperature shiftreactor is taken around a downstream low temperature shift reactor, theby-pass flow rate around the low temperature shift reactor may be in therange of 1-70% of the partially shifted gas or synthesis gas flow rate.Where there is a by-pass around all of the water-gas shift reactor(s),as the concentration of carbon monoxide is higher, the proportion thatis by-passed will be smaller. The by-pass flow rate in such cases may bein the range of 1-25% of the unshifted synthesis gas flow rate.

Where carbon dioxide constitutes a significant proportion of the shiftedor unshifted synthesis gas, a by-pass around the carbon dioxide removalunit, or all or part of the water-gas shift section and the carbondioxide removal unit, will be a smaller proportion of the total processgas flow, compared to a by-pass around all or part of the water-gasshift section only. If a by-pass stream is taken around a lowtemperature shift reactor and the carbon dioxide removal unit, theby-pass flow rate may be in the range of 1-20% of the total process gasflow. Similarly, if a by-pass stream is taken around all of thewater-gas shift section and the carbon dioxide removal unit, the by-passflow rate may be in the range of 1-15% of the unshifted synthesis gasflow rate.

It should be understood that in the process, the total concentration ofcarbon oxides (on a wet basis) is the same but the ratio of carbonmonoxide to carbon dioxide will change.

Steam present in the shifted gas mixture may be condensed by cooling theshifted gas to below the dew point using one or more heat exchangersfed, for example, with cooling water. The resulting dried shifted gasmay be passed to the carbon dioxide removal unit. The condensaterecovered may, if desired, be fed to a condensate stripping unit or, ifdesired be fed to steam generators that produce steam for the synthesisgas generation and/or water-gas shift stages.

A carbon dioxide removal unit is used to recover carbon dioxide from theshifted synthesis gas in step (c). It is located downstream of awater-gas shift stage, and upstream of the methanol synthesis stage. Anysuitable carbon dioxide removal unit may be used. Suitable removal unitsmay function by reactive or chemical absorption, such as those known asaMDEA™ or Benfield™ units that are based on using regenerable amine orpotassium carbonate washes, or by physical absorption, based on usingmethanol, glycol or another liquid at low temperature, such asRectisol™, Selexol™ units. Carbon dioxide removal may also be performedby means of pressure-swing adsorption (PSA) using suitable solidadsorbent materials. Such carbon dioxide removal apparatus and materialsare commercially available. Some or all of the carbon dioxide formed inthe synthesis gas may be removed to produce a gas stream comprisingmainly hydrogen and nitrogen with low levels of carbon monoxide and/orcarbon dioxide. The carbon dioxide removed by the carbon dioxide removalunit may be captured, treated to remove contaminants such as hydrogen,and stored or used for reaction downstream with the ammonia produced toform urea.

The portion of the synthesis gas produced by the synthesis gasgeneration unit may by-pass the carbon dioxide removal unit. The portionof the synthesis gas by-passing the carbon dioxide removal unit may alsoby-pass one or more of the upstream water-gas shift reactors or, theremay be no by-pass of the water gas shift stage, such that a portion of ashifted gas is by-passed around the carbon dioxide removal unit. In apreferred embodiment, a by-pass stream of partially shifted gas from ahigh temperature shift stage or a medium temperature shift stage istaken around a low temperature shift reactor and the carbon dioxideremoval unit.

As an alternative to a by-pass around the carbon dioxide removal unit,the operating conditions and effectiveness of the carbon dioxide removalunit may be adjusted so that the carbon dioxide is only partiallyremoved from the feed gas. Thus the carbon dioxide concentration in thecarbon dioxide-depleted gas stream fed to the methanol synthesis unitmay be controlled by manipulation of process variables within the carbondioxide removal unit. A by-pass stream around one or more of the watergas shift reactors may be used in combination with such manipulation toprovide the desired gas composition for the downstream processes.

It is desirable to remove water from the carbon dioxide-depletedsynthesis gas. Water removal, or drying, is desirable to protect thedownstream methanol synthesis catalyst, improve the rate of the methanolsynthesis reaction and to minimise water in the crude methanol product.Water removal may also improve the performance and reliability of thefirst stage of compression. Water removal may be accomplished by coolingthe water-containing gas below the dew point using one or more stages ofheat exchange and passing the resulting stream through a gas liquidseparator. Further stages of drying, e.g. with a desiccant may beperformed if desired.

Methanol is synthesised from the carbon dioxide-depleted synthesis gasin step (d). The synthesis reactions may be depicted as follows:

3H₂+CO₂

CH₃OH+H₂O

2H₂+CO

CH₃OH

Any methanol production technology may be used. Methanol is synthesisedin a synthesis unit, which may comprise a methanol converter containinga methanol synthesis catalyst. The process can be operated on aonce-through basis, or on a recycle basis in which unreacted productgas, after optional methanol condensate removal, is mixed with make-upgas comprising hydrogen and carbon oxides in the desired ratio andreturned to the methanol reactor. The methanol synthesis, because it isexothermic, may involve cooling by indirect heat exchange surfaces incontact with the reacting gas, or by subdividing the catalyst bed andcooling the gas between the beds by injection of cooler gas or byindirect heat exchange.

A crude methanol product comprising methanol, water and trace amounts ofimpurities such as ethanol may be recovered by cooling of the productgas stream to below the dew point, e.g. with cooling water. If desired,liquid ammonia may be used in a further cooling stage. Alternatively, orin addition, methanol may be recovered by scrubbing the product gas withwater.

Any methanol synthesis catalyst may be used, but preferably it is basedon a promoted or un-promoted copper/zinc oxide/alumina composition, forexample those having a copper content in the range 50-70% wt. Promotersinclude oxides of Mg, Cr, Mn, V, Ti, Zr, Ta, Mo, W, Si and rare earths.In the catalyst, the zinc oxide content may be in the range 20-90% wt,preferably 20-50% wt, and the one or more oxidic promoter compounds, ifpresent, may be present in an amount in the range 0.01-10% wt. Magnesiumcompounds are preferred promoters and the catalyst preferably containsmagnesium in an amount 1-5% wt, expressed as MgO. The synthesis gas maybe passed over the catalyst at a temperature in the range 200-320° C.,and at a pressure in the range 20-250 bar abs, preferably 20-120 barabs, and a space velocity in the range 500-20000 h⁻¹. Because the aim ofthe process is not to maximise methanol production, the inlettemperature of the methanol synthesis stage may be lower, e.g. 200-270°C. thus extending the catalyst lifetime by reducing sintering of theactive copper sites.

In the present process, a single stage of methanol synthesis issufficient. Nevertheless, if desired, the methanol synthesis may be partof a multiple stage synthesis process where the product gas, with orwithout condensate removal, is fed to one or more further methanolsynthesis reactors, which may contain the same or different methanolsynthesis catalyst. Such methanol production apparatus and catalysts arecommercially available. A purge gas stream may be removed to prevent theundesirable build-up of inert/unreactive gases. If desired methanol mayalso be synthesised from this purge gas, or hydrogen recovered from itto adjust the stoichiometry of the feed gas or to generate power.

The crude methanol recovered from the methanol synthesis unit containswater and other impurities that are often separated from the productmethanol by one or more stages of distillation. In the present process,preferably all of the recovered methanol is oxidised to produceformaldehyde. The formaldehyde production unit may use purified methanolas the feed or crude methanol as the feed. By “crude methanol” weinclude the liquid product of the methanol synthesis reactor afterseparation from the gas stream and a liquid methanol product in whichthe water content of the methanol has been adjusted to the range 5-20%by weight. When using crude methanol, the water content is desirably≥20% wt so that the formaldehyde stabiliser products are producedefficiently at suitable concentrations. This makes it possible to sendcrude methanol directly to the formaldehyde plant without the need formultiple distillation steps. Achieving low water content by controllingthe reactor inlet gas composition using the appropriate by-pass wouldsave both capital cost on distillation columns and associated equipment,as well as the operating cost of this equipment, resulting in asignificant benefit.

The crude methanol may be sent for storage in a suitable storage tank.Alternatively, the crude methanol may be subjected to one or morepurification stages, including a degassing stage, in a methanolpurification unit prior to feeding it to the oxidation reactor. Thedegassing stage or any distillation stages may be provided bydistillation columns heated using heat recovered from the oxidationreactor or elsewhere in the process. In particular, the degassing stagemay be heated using steam generated by the oxidation stage. Thissimplification of the purification offers significant savings in capitaland operating costs for the process.

Optionally, a water scrubber may be employed to improve recovery of thesynthesised methanol from the unreacted gas after bulk separation.Recovering the methanol will improve the efficiency of the plant andwill reduce the duty on the methanator. The wash water containingmethanol recovered from the water scrubber may be blended with the crudemethanol or may be sent to steam generators used to provide steam forthe synthesis gas generation and/or the water-gas shift stages.

The unreacted gas stream recovered from the methanol synthesis unitafter separation of the crude methanol stream is the methanol synthesisoff-gas. It comprises nitrogen, hydrogen, and small amounts of methane,argon, carbon monoxide and carbon dioxide.

Methanol is oxidised to formaldehyde in step (e). Any formaldehydeproduction technology using air as the oxidant may be used. Theformaldehyde is synthesised in a formaldehyde production unit, which maycomprise an oxidation reactor containing an oxidation catalyst. Theoxidation catalyst may be provided as a fixed bed or withinexternally-cooled tubes disposed within the reactor. A compressed airsource, which may be from the single source as described above, is usedin the formaldehyde production unit. The air may be in the temperaturerange 10-50° C. The air and methanol may be passed to the reactorcontaining an oxidation catalyst in which the methanol is oxidised. Airis preferably provided at 1.1-5 bar abs, e.g. from a first stage ofcompression of the air fed to the process. The amount of air fed to theformaldehyde production unit is a relatively small proportion of the airfed to the overall process and so compression costs are notsignificantly increased and may be more than compensated for by theremoval of additional compression equipment.

Production of formaldehyde from methanol and oxygen may be performedeither in a silver- or a metal oxide catalysed process operated atmethanol-rich or methanol-lean conditions, respectively. Hence theoxidation catalyst may be selected from either a silver catalyst or ametal oxide catalyst, preferably comprising a mixture of iron andmolybdenum oxides. Vanadium oxide catalysts may also be used. In themetal oxide process, the principal reaction is the oxidation of themethanol to formaldehyde;

2CH₃OH+O₂→2CH₂O+2H₂O

Over silver catalysts, in addition to the above oxidation reaction,methanol is also dehydrogenated in the principal reaction for this typeof catalyst;

CH₃OH→CH₂O+H₂

In the metal oxide process, formaldehyde is produced in multi-tubereactors. Typically, a reactor comprises 10-30,000 tubes containingcatalyst pellets or extrudates and cooled by oil or by molten salts asheat transfer fluid. Since the reaction is highly exothermic (ΔH=−156kJ/mol), isothermal conditions are difficult to obtain and consequentlya hotspot may be formed within the reaction zone. In order to limit thehot spot temperature, at the first part of the reactor the catalyst canbe diluted with inert pellets or extrudates. The catalyst used in theoxide process is preferably a mixture of iron molybdate Fe₂(MoO₄)₃ andmolybdenum trioxide MoO₃ with a Mo:Fe atomic ratio between 2 and 3. Theplant yield is high (88-94%) and neither molybdenum nor iron are toxic,which is favourable considering both environmental and human healthaspects.

Air is preferably used at levels to maintain the oxygen content at theinlet of the reactor below the explosive limit. The feed gas maytherefore comprise ≥6.5 vol % methanol for a once-through reactor orabout 8-11 vol % methanol where there is recirculation. The oxidationreactor may be operated adiabatically or isothermally, where the heat ofreaction can be used to generate steam. The inlet temperature to theoxidation reactor is typically in the range 80-270° C., with iron-basedcatalytic processes operating up to 400° C. and silver-based processesup to 650° C.

A single passage through the oxidation reactor can result in high yieldsof formaldehyde, or if desired it is possible to recycle unreactedgases, which comprise mainly of nitrogen, to the reactor inlet tomaintain a low oxygen concentration. Due to the scale required in thepresent process, preferably the formaldehyde production stage isoperated without recycle of oxidised gas to the inlet of the oxidationreactor as this removes the need for a recycle compressor and henceoffers further savings.

An absorption tower may be used to extract the formaldehyde product fromthe oxidised gas mixture into either water to produce aqueousformaldehyde solution, or a urea solution to produce a urea-formaldehydeconcentrate (UFC). The absorption tower may contain a selection ofpacking, trays and other features to promote the absorption, and coolingwater may be used to provide the product at a temperature in the range20-100° C. The absorption stage typically runs at a slightly lowerpressure than the reactor.

In the present process, products made from the formaldehyde are used tostabilise urea. The formaldehyde production unit may be used to producean aqueous formaldehyde solution (formalin) or a urea-formaldehydeconcentrate (UFC). Urea formaldehyde concentrate that may be usedtypically comprises a mixture of about 60%, wt formaldehyde, about 25%wt urea and the balance about 15% water. Such a product may be termed“UFC85”. Other UFC products may also be used, e.g. UFC80. Otherformaldehyde products may also be produced. Excess formaldehyde productsmay be recovered and sold.

The formaldehyde production unit generates a vent gas which may bepassed to a vent gas treatment unit such as an emission control unit oremission control system (ECS) and discharged to atmosphere. An emissioncontrol unit or system may comprise a catalytic combustor that reactsany carbon monoxide, methanol, formaldehyde and dimethyl ether in thevent gas with oxygen. The gas emitted from an ECS, i.e. an ECS effluent,may comprise carbon dioxide, steam and nitrogen and therefore may berecycled, preferably after suitable compression, to one or more stagesof the process. Thus the ECS effluent may be passed to the carbondioxide-removal stage where steam and carbon dioxide may be recovered,to provide additional nitrogen in the synthesis gas. Alternatively theECS effluent may be provided to the methanol synthesis stage where thecarbon dioxide may be reacted with hydrogen in the synthesis gas toproduce additional methanol. Alternatively, the ECS effluent may be fedto the urea production unit to provide carbon dioxide for additionalurea production.

In another embodiment, the vent gas treatment unit comprises agas-liquid separator that separates the nitrogen-rich off-gas fromliquid methanol, which may be recycled to the oxidation reactor directlyor after one or more stages of purification. The nitrogen-rich gasseparated in the separator may be compressed and passed to the ammoniasynthesis stage.

Alternatively the formaldehyde vent gas may be recycled directly to theprocess, i.e. the vent gas treatment unit or system may be omitted. Inone embodiment, the formaldehyde vent gas is recycled directly to thesynthesis gas generation unit as a fuel gas so that the organiccontaminants present in the vent gas may be combusted to generateenergy. The formaldehyde vent gas may, for example, be recycled directlyto the fuel gas stream of a primary reformer or may be fed to a furnacefor steam generation. In this way an ECS or vent gas treatment unit isnot required, which offers considerable savings. Alternatively the ventgas may be combined with a hydrocarbon feedstock fed to the synthesisgas generation unit.

Alternatively, the formaldehyde vent gas may be recycled directly to thecarbon dioxide removal stage so that the carbon dioxide and water vapourpresent in the vent gas may be captured. Organic contaminants such asmethanol, formaldehyde and dimethyl ether may also be captured, e.g.using a PSA unit.

Alternatively, the formaldehyde vent gas maybe recycled directly to themethanol synthesis stage. Direct recycling is simpler and is preferred.With direct recycling, the by-products will be limited by equilibriumacross the methanol synthesis catalyst and so will not accumulate inthis recycle loop. The nitrogen is also recovered without the need forcatalytic combustion or intensive pressurisation.

The formaldehyde vent gas may be recycled directly to one, two or moreof these alternatives.

The formaldehyde production unit may also produce an aqueous wastestream, for example a condensate recovered as a by-product of themethanol oxidation. This condensate may contain organic compounds suchas methanol, formaldehyde and dimethyl ether and therefore provide apotential source of hydrocarbon for the process. In one embodiment, theprocess condensate is recycled to the synthesis gas generation stagewhere it is used to generate steam for use in steam reforming. The steammay be formed in a conventional boiler and added to the hydrocarbon feedor may, preferably, be generated in a saturator to which the aqueouseffluent and hydrocarbon are fed. Alternatively the effluent may be fedto a process condensate stripper.

In the methanation stage (f), residual carbon monoxide and carbondioxide in the methanol synthesis off-gas stream is converted to methanein the methanator. Any suitable arrangement for the methanator may beused. Thus the methanator may be operated adiabatically or isothermally.One or more methanators may be used. A nickel-based methanation catalystmay be used. For example, in a single methanation stage, the gas fromthe methanol synthesis stage may be fed at an inlet temperature in therange 200-400° C., preferably 325-375° C., to a fixed bed of pelletednickel-containing methanation catalyst. Higher inlet temperatures areuseful to decompose any methanol in the feed gas. Such catalysts aretypically pelleted compositions, comprising 20-40% wt nickel. Suchmethanation apparatus and catalysts are commercially available. Thepressure for methanation may be in the range 10-80 bar abs or higher upto 250 bar abs. Steam is formed as a by-product of methanation. Thesteam is desirably removed using conventional means such as cooling andseparation of condensate. An ammonia synthesis gas stream may berecovered from the methanation and drying stage. Such methanationapparatus and catalysts are commercially available.

The methanated gas stream may be fed to the ammonia production unit asthe ammonia synthesis gas. However, the hydrogen:nitrogen molar ratio ofthe methanated gas stream may need to be adjusted, for example byaddition of nitrogen from a suitable source, to provide the ammoniasynthesis gas. The adjustment of the hydrogen:nitrogen molar ratio is toensure the ammonia synthesis reaction operates efficiently. The nitrogenmay be provided from any source, for example from an air separation unit(ASU). The adjustment may be performed by direct addition of nitrogen tothe methanated gas stream. The adjusted gas mixture may then be passedto the ammonia synthesis unit as the ammonia synthesis gas.

Ammonia is synthesised in step (g). The ammonia synthesis gas may becompressed to the ammonia synthesis pressure and passed to an ammoniaproduction unit. The ammonia production unit comprises an ammoniaconverter containing an ammonia synthesis catalyst. The nitrogen andhydrogen react together over the catalyst to form the ammonia product.Ammonia synthesis catalysts are typically iron based but other ammoniasynthesis catalysts may be used. The reactor may operate adiabaticallyor may be operated isothermally. The catalyst beds may be axial and/orradial flow and one or more beds may be provided within a singleconverter vessel. The conversion over the catalyst is generallyincomplete and so the synthesis gas is typically passed to a loopcontaining a partially reacted gas mixture recovered from the ammoniaconverter and the resulting mixture fed to the catalyst. The synthesisgas mixture fed to the loop may have a hydrogen:nitrogen ratio of2.2-3.2. In the ammonia production unit, the hydrogen/nitrogen mixturemay be passed over the ammonia synthesis catalyst at high pressure, e.g.in the range 80-350 bar abs, preferably 150-350 bar abs for large-scaleplants, and a temperature in the range 300-540° C., preferably 350-520°C.

A purge gas stream containing methane and hydrogen may be taken from theammonia synthesis loop and fed to the synthesis gas generation step orused as a fuel.

Compression of the synthesis gas is preferably effected in multiplestages, with a first and a second stage performed before the methanolsynthesis to achieve e.g. 50-100 barg, preferably 80-100 barg, and athird stage after methanation to achieve a higher pressure, e.g. 150-250barg, before the ammonia synthesis. Thus methanol synthesis may usefullybe provided between the second and third stages of compression, with themethanator downstream of methanol synthesis and upstream of the thirdstage of compression. Alternatively, the methanol synthesis may usefullybe provided upstream of the first stage of compression.

Urea is produced in step (h) by reacting ammonia from step (g) withcarbon dioxide recovered from step (c). Typically only a portion of theammonia produced in step (g) will be used to produce urea, which islimited by the amount of carbon dioxide recovered in step (c). Theexcess ammonia may be recovered and used to make nitric acid, ammoniumnitrate or ammonia products for sale. Any urea production technology maybe used. For example, ammonia and carbon dioxide may be combined in afirst reactor in the range 140-200° C. and 120-220 bar abs to formammonium carbamate as follows;

NH₃+CO₂

NH₂COONH₄

The ammonium carbamate is then dehydrated in a further reactor to formurea;

NH₂COONH₄

NH₂CONH₂+H₂O

The high pressure favours ammonium carbamate formation and the hightemperature favours the dehydration, so the resultant mixture containsall the above components. Unreacted carbamate is therefore generallydecomposed back to ammonia and carbon dioxide, which may then berecycled to the reactor. The carbon dioxide readily dissolves in thewater from the dehydration, which if recycled supresses the equilibriaand so the system may be run with excess ammonia to minimise thisrecycle. The decomposition and subsequent recycling can be carried outin one or more successive stages at decreasing pressures to minimise theultimate concentration of ammonium carbamate dissolved in the ureasolution. An alternative process arrangement uses the fresh carbondioxide gas to strip unreacted ammonia and carbon dioxide from theammonium carbamate and urea solution at the same pressure as thereactor. Further unreacted material is recycled from lower pressurestages as ammonium carbamate solution. Such urea production apparatus iscommercially available.

Formaldehyde-stabilised urea is produced in step (i) by mixing ureaproduced in step (h) stream and a stabiliser prepared using formaldehyderecovered from the formaldehyde production unit in step (e). Thestabiliser may be any formaldehyde-containing stabiliser; includingaqueous formaldehyde and an aqueous urea-formaldehyde concentrate.Aqueous formaldehyde and urea formaldehyde concentrate may be prepareddirectly in the formaldehyde production unit. Formaldehyde, either as aconcentrated solution or as a combined solution of urea and formaldehydemay be added to molten urea prior to forming into either prills orgranules. This reduces the tendency of the urea to absorb moisture andincreases the hardness of the surface of the solid particles, preventingboth caking (bonding of adjacent particles) and dusting (abrasion ofadjacent particles). This maintains the free flowing nature of theproduct; prevents loss of material through dust, and enhances thestability during long term storage. If urea is available then it ispreferable to use the urea formaldehyde solution as a stable solutionwith a higher formaldehyde concentration can be produced, whichminimises the water being added to the molten urea. Such stabilised ureaproduction apparatus is commercially available.

The present invention will now be described by way of example withreference to the accompanying drawings in which;

FIG. 1 is a schematic representation of a process according to a firstaspect of the present invention including by-pass of the one or morewater-gas shift reactors, and

FIG. 2 is a schematic representation of a process according to a secondaspect of the present invention including by-pass of the carbon dioxideremoval unit.

It will be understood by those skilled in the art that the drawings arediagrammatic and that further items of equipment such as reflux drums,pumps, vacuum pumps, temperature sensors, pressure sensors, pressurerelief valves, control valves, flow controllers, level controllers,holding tanks, storage tanks, and the like may be required in acommercial plant. The provision of such ancillary items of equipmentforms no part of the present invention and is in accordance withconventional chemical engineering practice.

In FIG. 1, a natural gas stream 10, steam 12 and an air stream 14 arefed to a synthesis gas generation unit 18 comprising a primary reformerand secondary reformer. The natural gas is primary reformed with steamin externally-heated catalyst filled tubes and the primary reformed gassubjected to secondary reforming in the secondary reformer with the airto generate a raw synthesis gas comprising nitrogen, hydrogen, carbondioxide, carbon monoxide and steam. A portion of the natural gas mayby-pass the primary reformer and be fed along with the primary reformedgas to the secondary reformer. A flue gas 16 is discharged from theprimary reformer. The steam to carbon monoxide ratio of the rawsynthesis gas may be adjusted by steam addition if necessary and the gassubjected to water-gas shift in a high temperature shift reactor 20containing a high temperature shift catalyst and then a low temperatureshift reactor 22 containing a low temperature shift catalyst. Thewater-gas shift reaction increases the hydrogen and carbon dioxidecontents and the steam and carbon monoxide contents are decreased. Theshifted synthesis gas is fed to a carbon dioxide removal unit 24operating by means of reactive absorption. A carbon dioxide and waterstream is recovered from the separation unit 24 by line 26 for furtheruse. A carbon dioxide-depleted synthesis gas 28 comprising hydrogen,carbon monoxide and nitrogen is passed from the carbon dioxide removalunit 24 to a methanol synthesis unit 30 comprising a methanol convertercontaining a bed of methanol synthesis catalyst. Upstream of themethanol synthesis unit 30, water in the shifted gas and the carbondioxide-depleted synthesis gas is removed by cooling and separation ofthe condensate. The dried carbon dioxide-depleted synthesis gas is thenheated and fed to the methanol synthesis unit. Methanol is synthesisedin the converter on a once-through basis, separated from the product gasmixture and recovered from the methanol synthesis unit 30. The methanolis passed via line 32 to a formaldehyde production unit 34 comprising anoxidation reactor containing an oxidation catalyst. An air source 36 isfed to the oxidation reactor where it is reacted with the methanol toproduce formaldehyde. The oxidation reactor is operated in a loop with aportion of the reacted gas fed to the inlet of the reactor. Theformaldehyde production unit is fed with cooling water 38 and generatesa steam stream 40 and a formaldehyde vent gas 42. The formaldehyde isrecovered in an absorption tower which may be fed with water or urea vialine 67 such that either aqueous formaldehyde or a urea-formaldehydeconcentrate (UFC) product stream 44 may be recovered from theformaldehyde production unit 34 for further use. A portion of theformaldehyde product stream 45 can be taken for use in, for example, aseparate urea-stabilisation plant or for sale, if the flow offormaldehyde produced is in excess of that required for the associatedurea plant. A methanol synthesis off-gas stream 46 comprising hydrogen,nitrogen and unreacted carbon monoxide recovered from the methanolsynthesis unit 30 is passed to a methanation unit 48 comprising amethanation reactor containing a bed of methanation catalyst. Carbonoxides remaining in the off-gas 46 are converted to methane and water inthe methanation reactor. Water is recovered from the methanation unit 48by line 50. The methanated off-gas is an ammonia synthesis gascomprising nitrogen, hydrogen and methane. The ammonia synthesis gas ispassed from the methanation unit 48 by line 52 to an ammonia synthesisunit 54 comprising an ammonia converter containing one or more beds ofammonia synthesis catalyst. The ammonia converter is operated in a loopwith a portion of the reacted gas fed to the inlet of the converter.Ammonia is produced in the converter and recovered from the ammoniasynthesis unit 54 by line 56. A purge gas stream 60 comprising methaneand unreacted hydrogen and nitrogen is recovered from the ammoniasynthesis unit 54 and provided to the synthesis gas generation unit 18as fuel and/or feed to the primary and/or secondary reformers. A ventgas stream 62 is also recovered from the ammonia synthesis unit 54. Aportion 58 of the ammonia is separated from the product stream 56. Theremaining ammonia is passed to a urea synthesis unit 64 where it isreacted with a purified carbon dioxide stream provided by stream 26 toproduce a urea stream and water. Water is recovered from the ureasynthesis unit 64 by line 66. The urea stream is passed by line 68 to astabilisation unit 70 comprising a stabilisation vessel where it istreated with aqueous formaldehyde or a urea formaldehyde concentrateprovided by line 44 to form a formaldehyde-stabilised urea product. Theformaldehyde-stabilised urea product is recovered from the stabilisationunit 70 by line 72.

In FIG. 1, in order to adjust the carbon oxides content of the gasstream 28 fed to the methanol synthesis unit 30, one or more by-passstreams are provided around the water-gas shift reactors. Thus in oneaspect, a by-pass stream 74 conveys a portion of the raw synthesis gasproduced by the synthesis gas generation unit 18 around the hightemperature shift reactor 20 to the feed to the low temperature shiftreactor 22. In an alternative aspect, a by-pass stream 76 conveys aportion of the high temperature shifted synthesis gas around the lowtemperature shift reactor 22 and to the feed stream to the carbondioxide removal unit 24. In a preferred aspect, a by-pass stream 76conveys a portion of the raw synthesis gas produced by the synthesis gasgeneration unit 18 around the high temperature shift reactor 20 and thelow temperature shift reactor 22 directly to the feed stream to thecarbon dioxide removal unit 24.

In FIG. 2, the synthesis gas generation, water-gas shift, carbon dioxideremoval, methanol synthesis, methanation, formaldehyde synthesis,ammonia synthesis, urea synthesis and stabilisation step are the same asdepicted in FIG. 1.

However, in FIG. 2, in order to adjust the carbon oxides content of thegas stream 28 fed to the methanol synthesis unit 30, one or more by-passstreams are provided around the carbon dioxide removal unit 24. Thus inone aspect, a by-pass stream 80 conveys a portion of the shiftedsynthesis gas produced by the low temperature shift reactor 22 aroundthe carbon dioxide removal unit 24 to the feed stream 28 to the methanolsynthesis unit 30. In an alternative aspect, a by-pass stream 82 conveysa portion of the high temperature shifted synthesis gas around the lowtemperature shift reactor 22 and the carbon dioxide removal unit to thefeed stream 28 to the methanol synthesis unit 30. In an alternativeaspect, a by-pass stream 84 conveys a portion of the raw synthesis gasproduced by the synthesis gas generation unit 18 around the hightemperature shift reactor 20, the low temperature shift reactor 22 andthe carbon dioxide removal unit 24 directly to the feed stream 28 to themethanol synthesis unit 30.

The present invention will now be described with reference to thefollowing example.

A process according to FIG. 1 was modelled to determine the effect ofby-passing a portion of the process gas 76 around the low temperatureshift reactor 22. The synthesis gas generation was by primary andsecondary steam reforming with air of natural gas with, both hightemperature and low temperature water-gas shift. There was nohydrocarbon by-pass of the primary reformer to the secondary reformer.The formaldehyde product was produced using the air oxidation of themethanol over a particulate iron/molybdenum catalyst disposed in cooledtubes, with recycle of a portion of the unreacted gas to control thetemperature within the oxidation reactor. The methanol synthesis wasperformed on a once though basis and the ammonia synthesis was performedwith recycle of a portion of the product gas to maximise ammoniaproduction. The compositions, pressures and temperatures for the variousstreams are given below.

Stream mole % dry 10 12 14 36 26 28 32 46 52 N₂ 1.32 78.08 78.08 0.1623.61 0.04 24.52 24.91 O₂ 20.96 20.96 H₂ 2.81 0.65 73.69 0.01 73.9073.48 NH₃ CH₄ 91.81 0.01 0.62 0.01 0.64 1.22 Ar 0.01 0.93 0.93 0.36 0.38CO₂ 2.56 0.03 0.03 99.15 0.10 0.01 0.03 CO 0.03 1.62 0.45 C₂H₆ 1.23 C₃H₈0.02 C₄+ 0.24 CH₃OH 99.92 0.08 CH₂O CO(NH₂)₂ Dry Flow 1525.0 1988.2240.2 1359.7 6658.1 78.2 6408.8 6308.7 kmol/hr H₂O 4369.4 151.1 18.32559.8 2.5 6.7 0.2 1.1 kmol/hr Total flow 1525.0 4369.4 2139.3 258.53919.5 6660.6 84.9 6409.0 6309.8 kmol/hr Temperature 365 390 25 225 10350 103 ° C. Pressure 44 45 1 1 92 3 90 182 bar abs Stream mole % dry 5668 67 44 42 72 76 N₂ <0.01 92.46 20.52 O₂ 5.49 H₂ <0.01 59.71 NH₃ 99.990.18 0.18 CH₄ 0.01 0.43 Ar 0.24 CO₂ 15.25 CO 3.85 C₂H₆ C₃H₈ C₄+ CH₃OH0.24 0.18 <0.01 CH₂O 0.04 0.04 82.50 0.02 1.10 CO(NH₂)₂ 99.78 99.7817.19 98.89 Dry Flow 3067.3 1225.6 14.8 84.8 200.6 1236.5 2299.2 kmol/hrH₂O 172.3 48.1 28.5 5.8 8.2 840.2 kmol/hr Total flow 3067.3 1397.8 62.9113.3 206.4 1244.7 3139.5 kmol/hr Temperature 22 133 45 30 30 95 205 °C. Pressure 17 1 4 3 1 1 32 bar abs

This example relates to a 1,200 mtpd ammonia plant that makes 86 mtpd ofUFC-85, for which about 60 mtpd of methanol is required. In this case(Case 1), the process characteristics are as follows;

Units Case 1 By-pass flow rate as % of the reformed synthesis mol % 30gas flow rate Carbon monoxide in by-pass mol % (dry) 3.85 Carbon dioxidein by-pass mo l% (dry) 15.25 Carbon monoxide inlet methanol synthesismol % (dry) 1.62 Carbon dioxide inlet methanol synthesis mol % (dry)0.01 Water content of crude methanol % wt 4.6 Relative size of methanolsynthesis catalyst bed % 100

The proportion of the gas flow that is diverted to the by-pass stream inCase 1 is relatively high as a large portion of the carbon oxides areremoved in the carbon dioxide removal unit. In comparison, Case 2,modelled according to FIG. 2, refers to the situation where there is aby-pass stream 82 around the low temperature shift reactor 22 and thecarbon dioxide removal unit 24. In comparison, Case 3, again modelledaccording to FIG. 2, refers to the situation where there is a by-passstream 84 around the high temperature shift reactor 20, the lowtemperature shift reactor 22 and the carbon dioxide removal unit 24.That is, a single by-pass stream around all three units, transferring aportion of the synthesis gas to the carbon dioxide-depleted gas stream.The model results are shown below.

Units Case 2 Case 3 By-pass flow rate as % of the reformed Mole % 7 6synthesis gas flow rate Carbon monoxide in by-pass mol % (dry) 3.8513.81 Carbon dioxide in by-pass mol % (dry) 15.25 7.12 Carbon monoxideinlet methanol mol % (dry) 0.70 1.27 synthesis Carbon dioxide inletmethanol synthesis mol % (dry) 1.29 0.53 Water content of crude methanol% wt 32.6 17.1 Relative size of methanol synthesis % 63 79 catalyst bed

By comparison to Case 1 above, the proportion of the process gas streambeing by-passed around the units is significantly smaller.

1. A process for producing formaldehyde-stabilised urea comprising thesteps of: (a) subjecting a synthesis gas comprising hydrogen, nitrogen,carbon monoxide, carbon dioxide and steam to one or more stages ofwater-gas shift in one or more water-gas shift reactors to form ashifted gas; (b) recovering carbon dioxide from the shifted gas in acarbon dioxide removal unit to form a carbon dioxide-depleted synthesisgas; (c) passing the carbon dioxide-depleted synthesis gas through amethanol synthesis unit to synthesize methanol and recovering themethanol and a methanol synthesis off-gas comprising nitrogen, hydrogenand residual carbon monoxide; (d) oxidizing at least a portion of therecovered methanol with air in a formaldehyde production unit; (e)methanating the methanol synthesis off-gas in a methanation reactorcontaining a methanation catalyst to form an ammonia synthesis gas; (f)synthesising ammonia from the ammonia synthesis gas in an ammoniaproduction unit and recovering the ammonia; (g) reacting a portion ofthe ammonia and at least a portion of the recovered carbon dioxide in aurea production unit to form a urea stream; and (h) stabilising the ureaby mixing the urea stream and a stabiliser prepared using formaldehyderecovered from the formaldehyde production unit, wherein a portion ofthe synthesis gas generated by the synthesis gas generation unitby-passes either the one or more water-gas shift reactors; the carbondioxide recovery unit; or the one or more water-gas shift reactors andthe carbon dioxide removal unit.
 2. The process according to claim 1wherein the synthesis gas is generated by steam reforming a hydrocarbonor gasifying a carbonaceous feedstock.
 3. The process according to claim1, wherein the synthesis gas is generated by adiabatic pre-reforming,primary reforming in a fired or gas-heated steam reformer and secondaryor autothermal reforming with air or oxygen-enriched air, or acombination thereof.
 4. The process according to claim 3, wherein aportion of the hydrocarbon by-passes the primary fired or gas-heatedsteam reformer.
 5. The process according to claim 1, further comprisingcompressing and dividing a source of air into a first portion that isprovided to the formaldehyde production unit of step (d) for oxidizingmethanol and a second portion that is further compressed and provided toa synthesis gas generation unit that contains the synthesis gas of step(a).
 6. The process according to claim 1, wherein the one or more stagesof water-gas shift of step (a) comprise one or more stages of hightemperature shift, low temperature shift, medium temperature shift,isothermal shift, or sour shift.
 7. The A process according to claim 1,wherein the shift stage of step (a) consists of a high temperature shiftreactor and a low temperature shift reactor and the portion of thesynthesis gas by-passes either the high temperature shift reactor, thelow temperature shift reactor or both the high temperature shift reactorand the low temperature shift reactor.
 8. The process according to claim1, wherein carbon dioxide is removed in step (b) using absorption oradsorption.
 9. The process according to claim 1, wherein the methanolsynthesis of step (c) is performed on a once-through basis, or on arecycle basis wherein unreacted gases, after methanol condensateremoval, are returned to the methanol synthesis unit.
 10. The processaccording to claim 1, wherein the methanol synthesis unit of step (c) isoperated in a single stage at an inlet temperature in the range of from200-320° C.
 11. The process according to claim 1, wherein crude methanolrecovered from step (c) is fed to oxidizing step (d).
 12. The processaccording to claim 11, wherein the crude methanol contains 20% wt orless of water.
 13. The process according to claim 1, wherein theformaldehyde production unit of step (d) comprises an oxidation reactorcontaining a bed of oxidation catalyst and is operated with or withoutrecycle of oxidised gas to an inlet of the oxidation reactor.
 14. Theprocess according to claim 1, wherein the formaldehyde production unitof step (d) generates a formaldehyde vent gas which is directly recycledor recycled after one or more stages of vent gas treatment in a vent-gastreatment unit.
 15. The process according to claim 14, wherein the ventgas treatment unit comprises a gas-liquid separator that separates thenitrogen-rich off-gas from liquid methanol.
 16. The process according toclaim 14, wherein the formaldehyde vent gas is recycled directly withouttreatment to the methanol synthesis step (c) or indirectly after firstpassing to an emission control unit comprising a catalytic combustor toconvert the vent stream into carbon dioxide, nitrogen and steam.
 17. Theprocess according to claim 14, wherein the formaldehyde vent gas isrecycled directly without treatment to the carbon dioxide removal step(b) or indirectly after first passing to an emission control unitcomprising a catalytic combustor to convert the vent stream into carbondioxide, nitrogen and steam.
 18. The process according to claim 14,wherein the formaldehyde vent gas is recycled after it has first passedto an emission control unit comprising a catalytic combustor to convertthe vent stream into carbon dioxide, nitrogen and steam to the ureasynthesis stage.
 19. The process according to claim 14, wherein theformaldehyde vent gas is recycled directly to a synthesis gas generationunit comprising the synthesis gas.
 20. The process according to claim 1,wherein the one or more stages of water-gas shift of step (a) comprise asingle stage of high temperature shift, a combination of hightemperature shift and low temperature shift, a single stage of mediumtemperature shift, or a combination of medium temperature shift and lowtemperature shift.